The present invention relates to comfort or climate control systems employed in the passenger compartment of automotive vehicles. Heretofore, where it has been desired to provide a way or means of controlling the discharge air temperature of an automotive heating and air conditioning system, temperature control has been accomplished by varying the opening of the water valve and controlling flow through the heater core or cabin heat exchanger to provide the desired temperature. It has been proposed in co-pending application Ser. No. 473,141 filed Feb. 1, 1990 now U.S. Pat No. 4,949,779 issued Aug. 21, 1990, entitled "Regulating Heater Discharge Air Temperature for Front and Rear Passengers in a Vehicle", Kenny et al and commonly owned with the present application, to provide automatic control of the heater core water valve by sensing the temperature of the discharge air over the cabin heat exchanger and the position of the valve and comparing with respect to a reference temperature level for generating a control signal to modulate the water valve. This latter type of system functions to regulate the heat exchanger discharge air temperature about a relatively selected level.
However, problems have been experienced in employing the aforesaid automatic discharge air temperature regulation system when the water pump is driven at engine idle speed, because at idle speed there is insufficient flow of heated liquid through the heat exchanger to enable the heat exchanger to provide adequate heat to the discharge air stream as may be needed for the desired regulation. Furthermore, if adequate flow of a liquid is provided at engine idle speed, then at higher engine RPM's encountered at road speeds, the water valve is operated in the nearly closed position by the system in order to maintain proper regulation. Consequently, it has been necessary for the sensitivity of the system, or resolution of the movement of the water valve to be prohibitively high in order to prevent over controlling or "hunting".
In the copending and commonly owned application Ser. No. 479,873 filed Feb. 14, 1990 now U.S. Pat. No. 4,974,664 issued Dec. 4, 1990, of Glennon et al entitled "Compensating For Water Pump Speed Variations In A Tempered Air System For Vehicle Passenger Compartments" it is described to sum signals representative of water valve position, water pump speed, relative selected temperature and Plenum air discharge temperature and generating a control signal to effect movement of the servo-actuator driven water valve until the sum is zeroed.
However, the aforesaid technique of Glennon et al has the disadvantage of requiring a water pump speed sensor located in the engine compartment and associated wiring to connect the electronic circuitry performing the electrical summing. Such an arrangement often requires additional wiring through the engine compartment bulkhead and is thus difficult to install and therefore costly in mass production.
Accordingly, it has been desired to provide a low cost and compact way or means of compensating for variations in water supply pressure for controlling liquid flow to a heat exchanger employed in a vehicle passenger compartment.